A northbound Brown line slows while it awaits for the southbound main tracks to clear....minutes, just a minute, or tens of seconds....it depends on which side you believe in the controversy about the flyover at Clark Tower.
Cleared, a northbound Brown line train leaves the interlocking and prepares to enter the branch. Because the NB move came from track 4, no other opposite move can be made except a SB Brown line move.
Clark Junction is called a "flat junction." Chicago elevateds have built flat junctions though its history. The first was on the west side near Marshfield, then three junctions bringing the four surrounding "L" companies onto the Loop. On the south side similar junctions were built at 59th St, Stewart Ave, and at Indiana Ave. On the north side, flat junctions were found at Clark Tower and at Howard terminal.
In 1938 the plans for flying junctions were included in the Initial Subway with interfaces with current "L" tracks and the subway lines. Provisions for additional junctions were included in the State St. tubes at Roosevelt and in the Dearborn St. tubes at Lake and Milwaukee. In the 1950s a flying junction brought the Douglas Park into the Congress median. In preparation in the Congress construction, a temporary flat junction was built at Paulina Ave on the Lake St. "L". Removed from service, the junction returned with the creation on the Pink line.
In the late 1960s the new Dan Ryan bought a flying junction at 18th St. The same trackage would be used when the Orange line came into being.